What are Gram negative bacteria?

Gram-negative bacteria (GNB) are a large group of bacteria that do not retain the crystal violet stain used in the Gram staining method.

The Gram staining method was a basic approach invented at the end of the 19th century to try to distinguish one bacteria from another, according to the chemical and physical properties of their cell walls.

What Gram staining intends to detect is the presence (Gram-positive) or absence (Gram-negative) of peptidoglycan in the bacterial outer layer.